People conveyors such as escalators or moving walkways which are formed from a series of adjacent moving steps generally include a passenger carrying path of travel which begins and ends at opposed landings, and a return path of travel which is disposed beneath the passenger carrying path of travel, and out of sight of persons using the conveyor. At each landing are disposed step reversal sprockets which engage step chains and guide the step chains and steps through a 180 arc to reverse the direction of movement of the steps. As the steps pass over the sprockets, the steps invert and reinvert their spacial orientation. With extensive usage and equipment aging, the possibility arises that a step may break loose from the step chain. A number of failures can produce this result. If a step should thus break loose, it will swing by gravity away from its normal path of travel, and the step tread will fall downwardly. When the steps are properly connected together on the step chain, there will be a constant procession of steps past any given point along the path of travel, and there will not exist any significant gaps in the step procession. When a step breaks loose, however, a significant gap will be created in the procession of steps. When a step breaks loose, the conveyor drive may continue to operate so that one using the conveyor would not know that a step is missing, or out of place. This can result in injury to passengers when the displaced step returns to the passenger carrying path of travel.
The problem of detecting abnormally positioned passenger conveyor steps has been addressed in the prior art. Japanese Kokai No. 53-140788 discloses a system for detecting abnormal step roller wear in a passenger conveyor which will result in misalignment of the steps. French Patent No. 082,074 discloses a control device including a proximity sensor for detecting recessed defects in the surface of a continuous conveyor. Russian Patent No. 850,541 discloses a monitor for an escalator for detecting the presence or absence of the escalator step rollers to detect detached escalator steps, should one occur. European Patent Publication No. 0,307,557 discloses an escalator step monitor which uses photoelectric detectors below the steps to detect the dropping of a step.